Microalgae are microscopic algae typically found in freshwater and marine systems. They are unicellular species which range from a few microns to a few hundred microns. Microalgae have also been identified as promising candidates for biodiesel fuel production and bio-based products because of their advantages of higher photosynthetic efficiency and faster growth compared to other energy crops, which accumulates a satisfactory amount of lipid for biodiesel production. However, microalgae harvesting still constitutes about 50% of the total cost for biofuel production from microalgae. Furthermore, a significant bottleneck in the application of microalgae for bio-based product development is the relatively low productivity of the culture. One fundamental reason for this is slow cell growth rates may be due to relative inefficient use of strong light.
The extraction of products from microalgae had been reported in literature. For example, Frenz et al. collected a substantial fraction of hydrocarbons (hydrophobic hydrocarbons) from microalga Botryococcus braunii (B. braunii) by exposing the cells to hexane for a short time. Frenz, J., et al, “Hydrocarbon Recovery By Extraction With A Biocaompatible Solvent From Free And Immobilized Cultures of Botryococcus Braunii”, Enzyme Microb. Technol. 1989, 11 717-724. In this method, the cells were harvested, separated and then contacted with the organic phase for extraction, and then the cells were returned to the bioreactor.
Sauer et al. applied a milking technique for the production of ectoines from the halophilic bacterium Halomonas elongata. Sauer et al, Bacterial Milking: A Novel Bioprocess For Production Of Compatible Solutes, Bioeng. 1998, 57, 306-313. Ectoines were biosynthesized in high salinity media. The cells were then transferred to a medium with low salinity and the ectoines were extracted. However, they did not compare productivity of the milking process with existing processes.
Recently, a new method was reported for milking β-carotene (a lipid) from Dunaliella salina in a two-phase bioreactor. Hejazi, M. A. et al. Selective Extraction of Carotenoids From The Microalga Dunaliella Salina With Retention of Viability. Biotechnol. Bioeng. 2002, 79, 29-36; Hejazi, M. A. et al. Milking Microalga Dunaliella Salina For β-carotene Production In Two-Phase Bioreactors, Biotechnol. Bioeng. 2004, 85, 475-481. In this method, cells are first grown under normal growth conditions and then stressed by excess light to produce larger amounts of β-carotene. At this stage, a biocompatible organic phase is added and the β-carotene is extracted selectively via continuous re-circulation of a biocompatible organic solvent through the aqueous phase containing the cells.
In open pond cultivation, at water depths of 0.15-0.20 meters, microalgae biomass concentrations are up to 1 g/L and productivities of 10-25 g/m2·d. Giuliano Dragone, Bruno Fernandes, António A. Vicente, and José A. Teixeira, Third Generation Biofuels From Microalgae, Current Research, Technology and Education Topics in Applied Microbiology and Microbial Biotechnology, A. Mendez-Vilas (Ed.), 2010, pp 1355. Given the relatively low biomass concentration (typically in the range of 1-5 g/L) obtainable in microalgae cultivation systems due to the limit of light penetration, solvent loss when scaling up and efficiency are two major concerns for this method. Organic solvents are detrimental to environment, and recycling of the extraction solvent is energy intensive.